Computerized Tomography (CT) scanners produce images of a subject by reconstruction of X ray attenuation data acquired over multiple view angles. Various corrections are applied to the measured raw data in order to obtain artifact free images which are true representation of the scanned subject. Some of these corrections are based on calibration processes, in which calibration measurements performed on the scanner are used to generate calibration tables and corresponding corrections.
One example of such calibration is known in the art as “air calibration”. Typically, air calibration involves performing a scan using a CT scanner, without there being a subject or phantom in the imaging space between the X ray source and the detector, so the detector array is irradiated by un-attenuated X ray beam. The acquired date, sometimes termed “air calibration data” is indicative of the relative efficiency and gain of the detector array elements and the variation in X ray beam intensity across the irradiation field. The air calibration data is used to normalize the attenuation data acquired during a subject scan.
Other effects that require calibration and correction are related to X ray beam hardening. Typical X ray sources used in CT scanners emit X radiation having a wide spectrum of energies. Since low energy X-ray photons are attenuated by matter more strongly than high energy photons, the spectrum of the radiation is changes as the radiation traverse the subject. The average X-ray energy of the transmitted beam increases for larger penetration lengths of the beam in the subject. Thus, the attenuation coefficient is relatively higher for thin than for thick paths. Since the thickness of the patient is different for different portions of a patient beam hardening gives rise to a phenomenon known as “cupping”, whereby an image of a cross section in a homogenous cylinder does not look like a flat disk but rather exhibits apparently lower attenuation values in the center.
Another phenomenon related to the beam hardening is a resulted imbalance between the output signals of different detector elements. The detector elements are normalized using air calibration data which are measured at one beam spectrum. However, the detection efficiency depends on the X-ray energy and this dependence is somewhat different from element to element. Therefore, different detector elements may response somewhat differently to X radiation of different spectra. Therefore, air calibration does not accurately normalize the attenuation data measured for a thick subject. In a third generation (rotate-rotate) CT scanner, imbalance between the output signals of neighboring detector elements results in artifacts known in the art as “rings”, “bands” and “bulls-eye”.
These phenomena and other non-linear effects are corrected in CT scanners by using calibration data derived from measurements on phantoms. Phantoms are objects of known composition and shape. One approach to the calibration known in the art and described e.g. in U.S. Pat. No. 4,352,020 to Horiba et al, is to scan a set of cylindrical phantoms of different diameters centered about the rotation center of the scanner and generate calibration tables based on the difference between the measured data and the data expected under ideal conditions. In subsequent subject scans, calibration tables are used to calculate correction factors depending on the attenuation level for each detector element. In order for the calibration process to be effective, the range of attenuations observed by each detector element in phantoms scans should generally correspond to the range of attenuation that would be observed by that detector element in a subject scan. For example, in a medical CT, detector elements in the center of the detector array typically observe higher attenuation than side detector elements. Calibration measurements on a set of cylinders made of organic polymer such as polypropylene, covering a range of diameters corresponding to small, medium and large human bodies would provide a satisfactory set of calibration tables.
U.S. Pat. No. 5,214,578 to Cornuejos et al. describes a method for calibration of a CT scanner using a cylindrical phantom placed off the isocenter (rotation axis) of the scanner. The advantages of phantom positioned off center are twofold: there is no need for tedious accurate positioning of the phantom at the center and the range of attenuation levels measured by each detector element during a rotational scan is increased, so measurements on a single phantom may potentially be sufficient. Yet, there are practical limitations in actual cases. Considering for example a scanner with a bore inner diameter of 600 mm, a cylindrical phantom of 400 mm diameter can be shifted at most by 100 mm from the bore center and the center detector elements are shaded by phantom thickness of 346-400 mm during a scanner rotation, not covering the lower attenuation levels expected to occur in clinical scans. Therefore, in certain cases the use of multiple phantoms may still be used. However, there is no teaching of how such phantoms are used.
U.S. Pat. No. 5,774,519 to Lindstrom et al, describes a system in which multiple phantoms are used. It appears that these phantoms are centered at the same position.
U.S. Pat. No. 6,848,827 to Xiaoye et al describes a system in which multiple cylindrical phantoms of different diameters are used. These phantoms are placed off the center of rotation by an amount that depends on the diameter of the phantom.
U.S. Pat. No. 6,944,258 to Nukui et al., all describe different methods for calculation and application of image corrections based on calibration measurements based on one or multiple phantoms positioned at a same position off the isocenter.
U.S. Pat. No. 7,149,277 to Tanigawa et al discloses a method for calculating correction coefficients using measurements on phantom having an oblong cross section or a cross section of an annular sector, from multiple directions. Also in this case the calibration may involve measurement of several phantoms of different sizes and shapes. Phantoms of oblong cross section have the advantage of presenting different penetration length from different view angles and thus increasing the range of attenuation levels observed during a rotational scan.
U.S. Pat. No. 6,148,057 to Urchuk et al. proposes a different approach, wherein the calibration of differences between detector elements are determined by measurements on slab absorbers of several thicknesses. The slab thickness may vary by stacking layers or by incrementing forward a step like phantom. These measurements do not involve rotation of the CT gantry but rather acquisition from a direction substantially normal to the slabs. Variations of this method include using an absorber of variable thickness respective the azimuthal direction of the scanner fan beam rather than a flat slab.
In any of the calibration methods it is desired to calibrate the scanner at the same X-ray attenuation range that is generally observed subject scans. For human body imaging, the center of the detector should be calibrated at the attenuation range approximately equivalent to absorption of 150 mm to 400 mm of water. Further, it is desired to use calibration phantoms with a chemical composition similar to that of a human body so that beam hardening effects in the phantom will simulate the beam hardening effects in a patient properly.
In modern CT scanners the number of detector rows in the axial direction is generally larger than in the past. Commercially available CT scanners with pixilated two dimensional detector arrays currently have up to 320 rows of detectors, covering an axial length of 160 mm at the isocenter of the scanner and over 200 mm at the periphery of the field of view. Other CT scanners known as “cone beam scanners” use a flat panel detector as the detector array and the axial coverage may be even higher. Therefore a calibration process based on a set of cylindrical or slab phantoms would require the use of cylinders, oblong objects or slabs, each over 200 mm thick in the axial direction. As a result, calibration phantoms according to any of the methods known in the art applied to wide beam CT scanners are heavy, difficult to handle and expensive.